Temperature sensing material type thermal use

ABSTRACT

In the present invention, a physical and chemical property of thermosensitive material is noted in selecting and using thermosensitive material to provide a noble and improved thermal fuse using thermosensitive material. To achieve this object, the present thermal fuse includes: a thermosensitive material formed of thermoplastic resin fusing at a prescribed temperature; a cylindrical enclosure accommodating the thermosensitive material; a first lead member attached at one opening of the enclosure, forming a first electrode; a second lead member attached at the other opening of the enclosure, forming a second electrode; a movable conductive member accommodated in the enclosure and engaged with the thermosensitive material; and a spring member accommodated in the enclosure, and pressed against and thus acting on the movable conductive member. When the thermosensitive material fuses at an operating temperature an electrical circuit between the first and second electrodes is switched.

TECHNICAL FIELD

The present invention relates generally to thermal fuses accommodatingthermosensitive material in a cylindrical enclosure and allowing aspring to act thereon to interrupt a circuit at a predeterminedtemperature or allow the circuit to conduct at the predeterminedtemperature, and particularly to thermal fuses employing thermosensitivematerial formed of material selected to provide the thermosensitivematerial with enhanced workability and durability.

BACKGROUND ART

A thermal fuse has widely been used in a variety of electric homeappliances, mobile equipment, communication equipment, businessequipment, vehicle-mounted equipment, AC adapters, chargers, motors,batteries and other electronic components as a protective componentaccurately detecting abnormal overheating of the equipment to rapidlyinterrupt a circuit or allow the circuit to conduct. Conventionally,thermal fuses have been categorized mainly in two types depending on thefuse element or thermosensitive material used: a thermal fuse usingconductive, low-melting fusible alloy; and a thermal fuse usingnon-conductive, thermosensitive material. These fuses are both aso-called non-reset thermal switch operating in response to anabnormally increasing ambient temperature to interrupt equipment'scurrent or provide a current path with a conducting state to protect theequipment. It operates at a temperature determined by thethermosensitive material used. Typically, it is offered in products as aprotective component functioning at a temperature ranging from 60° C. to250° C. on a rated current ranging from 0.5A to 15A and acts as anelectrical protection means allowing an initial conducting or interruptstate for ordinary temperature to be inverted at a predeterminedoperating temperature to provide an interrupt or conducting state.

The thermal fuse using non-conductive thermosensitive material istypically configured as follows: A cylindrical enclosure has oppositeends each with a lead attached thereto and an organic chemical agenthaving a prescribed melting point is molded into a predeterminedgeometry to obtain a thermosensitive material which is then accommodatedin the enclosure and for which a compression spring or the like exertsforce on a movable conductor to configure the fuse. For example,Japanese Patent Laying-Open No. 10-177833 describes a thermal fusehaving an enclosure in the form of a glass tube which has an internalportion provided with a pair of conductive films and accommodatessuccessively a thermosensitive material, a conductor movable between aconducting position and an interrupt position, and a compression springexerting force on the movable conductor with an insulator posedtherebetween.

Japanese Patent Laying-Open No. 5-307925 describes that a cylindricalenclosure in the form of a metallic casing is used with one opening'slead crimped and thus fixed and the other opening's lead fixed via aninsulated bushing and thermosensitive material is introduced into thecasing between two spring plates and at room temperature the springplates are brought into contact with an internal wall surface of thecasing so as to provide a simply structured thermal fuse. Furthermore,Japanese Patent Laying-Open No. 9-282992 describes a thermal fuseemploying a metallic casing which has one end with a leading leadelectrically connected and receives an elastic body, a thermosensitivematerial, a piece of metal, a movable piece in the form of a spring, andsealing ceramic successively, the ceramic being penetrated by the otherlead. Furthermore, Japanese Patent Laying-Open Nos. 5-135649 and11-111135 describe a well-known thermal fuse using a spring member inthe form of a strong compressing spring and a weak compression spring toexert force on a movable contact body to ensure a movement for anoperating temperature.

The thermal fuses using thermosensitive material as described aboveemploy a relatively pure organic chemical for the thermosensitivematerial. More specifically, this substance is granulated and moldedinto a predetermined form to provide the thermosensitive material. Itis, however, susceptible to the material's softening, deformation,sublimation, deliquescent property and other surrounding, environmentalconditions and there have been a large number of concerns in terms ofmanagement of production steps, conditions for storing the finishedproduct, and the like. For example, Japanese Patent Laying-Open No.2-281525 describes that a residual stress introduced when a casingaccommodating thermosensitive material and an external leading lead arecrimped and thus fixed introduces a gap, which allows external moistureto enter the casing and negatively affect the thermosensitive material.When thermosensitive material having deliquescent property is exposed toexternal air, the material deforms, sublimate and the like. Accordinglyin molding such thermosensitive material a complete management ofsealing is required to block external air.

Furthermore, a mold is small in mechanical strength such as hardness. Assuch, when a thermal fuse is being fabricated a spring's force candeform the mold, resulting in a defect. Furthermore, if a completedthermal fuse is stored at high temperature in high humidity thethermosensitive material sublimates, deliquesces and the like, which canaffect the product's longevity and also impair its electricalcharacteristics. Conventional thermosensitive material employing organicchemical, in particular, when it is exposed to high temperature,significantly softens and deforms. It thus diminishes, resulting in acontact dissociating disadvantageously. Accordingly there has been aneed for a thermal fuse using thermosensitive material that is lessaffected in use by its surrounding environment, chronological variationand the like and also have the thermosensitive material free of defectwhen the fuse is stored in severe atmosphere, exposed to hightemperature and high humidity, toxic gas, and the like.

DISCLOSURE OF THE INVENTION

The present invention has been proposed to resolve the abovedisadvantages. The present invention notes thermosensitive material'sphysicochemical property in selecting and using thermosensitive materialso as to provide a noble and improved thermal fuse employingthermosensitive material.

In accordance with the present invention the thermosensitive materialformed of thermoplastic resin is selected and used. In the selection, aphysicochemical property is considered to select a material having aproperty that allow the material to readily be molded and handled in theproduction process and can also address the mold's alteration,deformation and the like. As a result, there is provided a thermal fuseemploying thermosensitive material that has an improved physicochemicalproperty and steady operating characteristics. More specifically, thereis disclosed a thermal fuse employing thermosensitive material,including: a thermosensitive material formed of thermoplastic resinfusing at a prescribed temperature; a cylindrical enclosureaccommodating the thermosensitive material; a first lead member attachedat one opening of the enclosure, forming a first electrode; a secondlead member attached at the other opening of the enclosure, forming asecond electrode; a movable conductive member accommodated in theenclosure and engaged with the thermosensitive material; and a springmember accommodated in the enclosure and pressed against the movableconductive member to act on the movable conductive member, and when thethermosensitive member fuses, the thermal fuse between the first andsecond electrodes switches to an interrupt state or a conducting state.

In particular, it is proposed that the thermosensitive member's mainmaterial is formed of thermoplastic resin mixed with an additiveproviding desired physicochemical properties, e.g., a filler formed ofan inorganic substance to enhance electrical characteristics includinginsulation resistance, dielectric strength and the like, an agentimproving mechanical properties including moldability, strength and thelike, and an agent improving chemical properties includinganti-oxidation or anti-aging. This can reduce deformation and alterationintroduced in thermosensitive material using organic chemical asconventional. The present thermal fuse employing thermoplastic resinthat provides steady operating characteristic can thus be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1A is a longitudinal cross section of a thermal fuse usingthermosensitive material in accordance with the present invention atroom temperature, and FIG. 1B is a longitudinal cross section of thethermal fuse employing thermosensitive material of the present inventionat an abnormally increasing temperature.

BEST MODES FOR CARRYING OUT THE INVENTION

The present thermal fuse employing thermosensitive material is, forexample, as shown in FIG. 1A, formed of a thermosensitive material 3formed of thermoplastic resin fusing at a prescribed operatingtemperature, a cylindrical metallic casing corresponding to acylindrical enclosure 1 accommodating thermosensitive material 3, afirst lead member 2 crimped and thus attached to one opening of thecasing and allowing the casing's internal wall surface to be a firstelectrode, an insulated bushing 9 arranged adjacent to the other openingof the casing, a second lead member 10 penetrating bushing 9 andallowing an end thereof to serving as a second electrode, a movablecontact corresponding to a movable, conductive member 7 accommodated inthe casing and electrically connected to the casing's internal wall, anda spring member 6, 8 accommodated in the casing and engaged with andexerting force on the movable contact. When the thermosensitive materialfuses, between the first and second electrodes a switch is made to aninterrupt state or a conducting state.

Note herein that in the present thermal fuse, “a switch is made to aconducting state ” implies both that the thermosensitive material havingreached its melting point exerts a load to interrupt a circuit and thatthe thermosensitive material having thermally deformed exerts a load tointerrupt the circuit. Preferably, the compression spring member isformed of a strong compression spring and a weak compression spring, andthe former resists the latter's resilience to press the movable contactagainst the second electrode. In particular, the strong compressionspring has opposite ends arranged between the thermosensitive materialand the movable contact with respective pressing plates posedtherebetween to facilitate fabrication and also provide steady springoperation and when the thermosensitive material fuses the weakcompression spring's force allows the movable contact to be moved tointerrupt a circuit so as to provide a thermal fuse that is normallyturned on and is turned off in abnormal condition. On the other hand,the strong compression spring can be integrated with the thermosensitivematerial and arranged in compressed condition. When the thermosensitivematerial fuses, the strong compression spring acting against force ofthe weak compression spring moves the movable contact to allow thecircuit to conduct so as to provide a thermal fuse employingthermosensitive material that is normally turned off and is turned on inabnormal condition.

The thermoplastic resin selected to form the thermosensitive material isgeneral-purpose plastic, engineering plastic or the like includingpolyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinylalcohol (PVA), polyvinylidene chloride (PVDC), polyethyleneterephthalate (PET) or similar general-purpose thermoplastic resin, orpolyamide (PA), polyacetal (POM), polycarbonate (PC), polybutyleneterephthalate (PBT), polyvinylidene fluoride (PVDF), polyphenylenesulfide (PPS), polyamidoimide (PAI), polyimide (PI)polytetrafluoroethylene (PTFE) or similar engineering thermoplasticresin and fluororesin, having a melting point corresponding to apredetermined operating temperature, and having a physicochemicalproperty desired as required. Furthermore, if necessary, two or moretypes of thermoplastic resin can be combined for use.

More specifically, for an operating temperature of 165° C., polyacetal(POM) resin having a melting point equal to the operating temperature isselected, and for an operating temperature of 220° C.,polybutylene-terephthalate (PBT) resin having a melting point close tothe operating temperature is selected. The present invention ischaracterized by a thermal fuse using thermosensitive material ofthermoplastic resin, and preferably an approach to improve desiredcharacteristics that depends on the thermoplastic resin'sphysicochemical property is taken. For example, if the resin chemicallyreadily oxidizes, ages and the like, an anti-oxidant, an anti-agingagent and the like are preferably mixed together. They are for example2,6-di-tert-butyl-p-cresol, butylated hydroxy anisole,2,2′-methylene-bis-(4-ethyl-6-tert-butyl phenol),1,1,3-tris-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,dilaurylthiodipropionate, dimyristylthiodipropionate, triphenylphosphate and the like. For example, if the thermosensitive material ispolyethylene, adding 2,6-di-tert-butyl-p-cresol in an amount of 0.001 to0.1% by mass is effective.

Furthermore, if the thermosensitive material's process or the materialthat has been processed is unsatisfactory in mechanical strength orelectrical insulation-related physical properties, a filler formed of aninorganic substance is preferably added thereto. The filler isadvantageously used in improving electric resistance, insulation and thelike. The inorganic filler for example includes alumina, silica, calciumsilicate, aluminium silicate, carbon black, calcium carbonate, magnesiumcarbonate, kaolin, talc and the like. Alumina and silica are preferableas they enhance insulation resistance or dielectric strength.

A feature of the present thermosensitive material employingthermoplastic resin is that it can be readily processed and it providesstrength larger than thermosensitive material using an organic chemicalas conventional. Conventionally, a chemical has been granulated and thentabletted to provide thermosensitive material. Using thermoplastic resinallows injection molding or extrusion to be used to provide massproduction inexpensively. In addition, thermosensitive material ofthermoplastic resin hardly softens, deforms or deliquesces at hightemperature in high humidity due to moisture or sublimates, as is oftenraised as an issue for the thermosensitive material using an organicchemical. This can not only facilitate storage before incorporation butresolve the thermosensitive material's diminishment with time and anassociated defect of a switch function.

The present thermal fuse using thermosensitive material in anotherembodiment includes a thermosensitive material formed of a thermoplasticresin fusing at a prescribed temperature, a cylindrical, metallic casingaccommodating the thermosensitive material, a first lead member crimpedand thus fixed to one opening of the casing and allowing the casing'sinternal wall surface to serve as a first electrode, an insulatedbushing arranged adjacent to the other opening of the casing, a secondlead member penetrating the bushing and having an end to serve as asecond electrode, and two flat plates in the form of tongues extendinglengthwise and having conductance and resilience arranged between thefirst and second electrodes. The two flat plates sandwich thethermosensitive material and have a rear surface brought into contactwith the casing's internal wall surface and when the thermosensitivematerial fuses the flat plates are narrowed to provide a non-contactcondition.

In still another embodiment, a thermal fuse using thermosensitivematerial is also disclosed as follows: a cylindrical, insulated tubeaccommodates thermosensitive material. First and second lead members arefixed to the tube's openings, respectively, and also electricallyconnected to first and second electrodes formed at an internal wallsurface of the casing. A conductor movable from a conduction position toan interrupt position of the first and second electrodes is accommodatedin the tube and pressed against the thermosensitive material via aninsulator by a spring arranged at one end of the tube.

FIRST EXAMPLE

FIGS. 1A and 1B show a thermal fuse using thermosensitive material ofthe present example. FIG. 1A is a cross section thereof at roomtemperature as normal, and FIG. 1B is a cross section of the thermalfuse in operation when it is abnormally heated. The present thermal fuseis configured of: a cylindrical, metallic casing corresponding to anenclosure 1 formed of copper, brass or similarly good conductor andpresenting satisfactory thermal conductance; a first lead member 2crimped and thus fixed to one opening of the casing; a switch functioncomponent including a thermosensitive material 3, a pair of pressingplates 4 and 5, a spring member 6 in the form of a strong compressionspring, and a movable, conductive member 7 in the form of a movablecontact formed of silver alloy satisfactorily conductive and adequatelyresilient, and a spring member 8 in the form of a weak compressionspring, all accommodated in the casing; an insulated bushing 9 insertedinto the other opening of the casing; and a second lead member 10penetrating bushing 9 and thus insulated from the casing.

Furthermore, a fixed contact 11, located at an inner end of the secondlead member 10, is brought into contact with the movable contact at roomtemperature, as shown in FIG. 1A, and spaced therefrom, as shown in FIG.1B, when temperature abnormally increases. Furthermore, a resin seal 12seals the casing's opening, bushing 9 and the second lead member 10.Furthermore, an insulated bushing 13 sufficiently raises resin seal 12at the casing's opening for sealing. Herein, thermosensitive member 3 isformed mainly of thermoplastic resin and molded, and a material whichfuses at a prescribed temperature at which the thermal fuse operates isselected and used. Furthermore, utilizing excellent thermosensitivematerial's strength can eliminate pressing plate 4 and still similarlyallow the fuse to be stored for a long a period of time and the absenceof plate 4 also allows a quick-response thermal fuse.

SECOND EXAMPLE

The present invention in another example provides a thermal fuse havinga simple structure using a thermosensitive material of thermoplasticresin, as described hereinafter. This thermal fuse includes, similarlyas has been described in the previous example, a thermosensitivematerial formed of thermoplastic resin fusing at a particular operatingtemperature, a cylindrical metallic casing accommodating thethermosensitive material, a first lead member crimped and thus fixed toone opening of the casing and allowing the casing's internal wallsurface to serve a first electrode, an insulated bushing inserted intoand thus fixed to the other opening of the casing, and a second leadmember penetrating the bushing and having an end serving as a secondelectrode, and further includes two flat plate springs sandwiching thethermosensitive material to provide both the function of a movableconductive member and that of a spring member, the flat plate springbeing arranged between the first electrode corresponding to the internalwall surface of the casing and the second electrode corresponding to anend of the second lead member.

More specifically, the flat plate springs formed of two pieces in theform of tongues having conductance and resilience and extendinglengthwise have one end fixed and electrically and mechanically coupledwith the second electrode of the second lead member and the other endformed of two pieces, opened desirably, and between the flat plates atthe opened side the thermosensitive material is inserted and thus allowsthe spring to exert force to hold the thermosensitive material andsimultaneously the flat plates have their rear surfaces brought intocontact with the first electrode of the internal wall surface of thecasing. As such, at normal, room temperature a conducting state ismaintained via the flat plate spring, and when the ambient temperaturereaches a particular temperature or more the thermosensitive materialfuses and the flat plate spring is thus compressed and disengaged fromthe casing's internal wall surface to interrupt an electrical circuitbetween the first and second lead members.

It has been confirmed that when the thermoplastic resin is for examplepolyacetal (POM) resin or polybutylene-terephthalate (PBT) resin, thethermal fuse operates, for ten samples, at 160.5 to 162.5° C. and 225 to227° C., respectively, and that as a variation in operation, ΔT=2° C. Inthis example, in reducing the number of components of the thermal fuseto simplify its structure, the strength of the thermoplastic resin ofthe thermosensitive material is effectively exhibited.

THIRD EXAMPLE

In the present example, a thermal fuse using thermosensitive material isconfigured as follows: A cylindrical insulated tube accommodatesthermosensitive material. First and second lead members are fixed to thetube's openings, respectively. First and second electrodes are formedeach at a portion of an internal wall surface of the casing. A sphericalconductor movable from a conducting position to an interrupt position ofthe first and second electrodes is accommodated in the tube. Thespherical conductor is pressed by a spring toward the thermosensitivematerial with a spherical insulator posed therebetween. The spring isarranged at one end of the tube and presses the spherical conductoragainst the thermosensitive material via the spherical insulator. Asnormal, the conductor is in contact with the internal wall surface'sfirst and second electrodes and positioned to maintain a circuit'sconduction state. As temperature increases and the thermosensitivematerial's temperature exceeds a particular temperature, thethermosensitive material fuses and thereby the conductor is moved by thespring's force to the interrupt position to interrupt the circuit. Thisexample is also simplified in structure and a thermosensitive materialof thermoplastic resin advantageous in strength is effectively utilized.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, thermosensitive material canbe selected from a wide range of thermoplastic resin and relativelyinexpensively offered, and, as required, an additive can be used toalter physical and chemical properties to provide enhanced moldability,prevent the molded thermosensitive material from deformation andalteration, and achieve increased longevity and stable operation. Inparticular, the ready fabrication and the thermosensitive material'simproved strength can help simplify components of the thermal fuse usingthe thermosensitive material to offer an inexpensive product.Furthermore in connection with its storage and chronological variation,the thermal fuse that does not use any organic chemicals as conventionalcan be stable for a long period of time even in high humidity, a toxic,gaseous ambient or the like. It can be protected from erosion and freeof impaired insulation level, and not only in storage but also in use itcan prevent degradation in performance including electricalcharacteristics and also reduce chronological variation to provide asignificant, practical effect such as helping to improve stability andreliability allowing operation constantly at a prescribed temperatureaccurately.

1. A thermal fuse employing thermosensitive material, comprising: athermosensitive material formed of thermoplastic resin fusing at aprescribed temperature; a cylindrical enclosure accommodating saidthermosensitive material; a first lead member attached at one opening ofsaid enclosure, forming a first electrode; a second lead member attachedat the other opening of said enclosure, forming a second electrode; amovable conductive member accommodated in said enclosure and engagedwith said thermosensitive material; and a spring member accommodated insaid enclosure and pressed against said movable conductive member to acton said movable conductive member, said thermosensitive member fusing atan operating temperature to switch an electrical circuit located betweensaid first and second electrodes.
 2. The thermal fuse of claim 1,wherein said thermosensitive member is formed by molding thethermoplastic resin mixed with an additive providing the thermoplasticresin with a desired physical or chemical property.
 3. The thermal fuseof claim 2, wherein said additive is a filler formed of an inorganicsubstance to alter said thermosensitive material's insulationresistance, dielectric strength or a similar electrical characteristic.4. The thermal fuse of claim 2, wherein said additive is an agentimproving said thermosensitive material's moldability, strength and thelike or an anti-oxidant or an anti-aging agent to alter mechanical orchemical property.
 5. The thermal fuse of claim 1, wherein saidcylindrical enclosure is a metallic, cylindrical casing having an endwith an opening receiving an insulated bushing, said first lead memberis crimped, fixed to and thus electrically and mechanically coupled withone opening of said metallic casing and also has said first electrodeformed at an internal wall surface of said casing, said second leadmember penetrates said bushing and is attached at the other opening ofsaid metallic casing insulatively, and has an end provided with saidsecond electrode, said movable conductive member is a contact movable asdesired between said first and second electrodes, and wherein saidspring member is a compression spring member engaged with said movablecontact.
 6. The thermal fuse of claim 5, wherein said compression springmember is formed of a strong compression spring and a weak compressionspring and wherein when said thermosensitive member does not operate,said strong compression spring acts against said weak compressionspring's resilience to allow said movable contact to abut against saidsecond electrode.
 7. The thermal fuse of claim 6, wherein said strongcompression spring is arranged between said thermosensitive material andsaid movable contact and when said thermosensitive material fuses, saidweak compression spring's force moves said movable contact to interrupta circuit.
 8. The thermal fuse of claim 6, wherein said compressionspring is arranged, as compressed by said thermosensitive material, andwhen said thermosensitive material fuses, said compression spring actsagainst said weak compression spring's force to move said movablecontact to allow a circuit to conduct.
 9. The thermal fuse of claim 1,wherein said cylindrical enclosure is a metallic, cylindrical casinghaving an end with an opening sealed by an insulated bushing, said firstlead member is crimped, fixed to and thus electrically and mechanicallycoupled with one opening of said metallic casing and also forms saidfirst electrode at an internal wall surface of said casing, said secondlead member penetrates said bushing and is attached at the other openingof said metallic casing insulatively and at an end thereof forms saidsecond electrode, said movable conductive member and said spring memberare two flat plates in a form of tongues extending lengthwise and havingconductance and resilience arranged between said first and secondelectrodes, said two flat plates sandwich said thermosensitive member toachieve contact with said metallic casing's internal wall surface andwhen said thermosensitive member fuses, a spacing between said flatplates is reduced to achieve a non-contact condition.
 10. The thermalfuse of claim 1, wherein said cylindrical enclosure is a cylindrical,insulated tube, said first and second lead members are fixed to saidtube's openings, respectively and also form said first and secondelectrodes, respectively, at said tube's internal wall surface, saidmovable conductive member is a conductor movable from a conductingposition of said first and second electrodes to an interrupt position ofsaid first and second electrodes, said spring member is arranged at oneend of said tube, and via an insulator said conductor is pressed againstsaid thermosensitive material.